Tissue transglutaminase is a multifunctional BH3-only protein

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Tissue transglutaminase (TG2) protein is found at high levels during early apoptosis.
• The Bcl-2 family regulates apoptosis through interactions between pro- and anti-apoptotic members.
• BH3-only proteins are key initiators of the intrinsic apoptosis pathway.

Purpose of the Study:

• To investigate the role of TG2 in apoptosis.
• To determine if TG2 contains a functional BH3 domain.
• To elucidate the mechanism by which TG2 induces cell death.

Main Methods:

• Bioinformatic analysis of TG2 primary sequence for homology to Bcl-2 family domains.
• Use of cell-permeable peptides mimicking the identified TG2 domain.
• Assessment of Bax conformational change, mitochondrial integrity, cytochrome c release, and cell viability.
• Co-immunoprecipitation assays to study protein interactions.
• Analysis of TG2 localization in TG2-overexpressing cells.

Main Results:

• TG2 possesses an eight amino acid domain with 70% identity to the Bcl-2 family BH3 domain.
• TG2-mimicking peptides induced Bax activation, mitochondrial dysfunction, cytochrome c release, and cell death.
• TG2 and its BH3 peptides directly interacted with Bax, but not with Bcl-2 or Bcl-X(L).
• Mutations in the TG2-BH3 domain abolished its ability to sensitize cells to apoptosis.
• TG2 localizes to the outer mitochondrial membrane and cross-links Bax during apoptosis induction.

Conclusions:

• TG2 functions as a novel, large, enzymatically active BH3-only protein.
• TG2 initiates apoptosis through direct interaction with Bax, leading to mitochondrial outer membrane permeabilization.
• TG2 represents a new subgroup of BH3-only proteins with significant implications for understanding apoptosis regulation.